Probably not actually, as apart from the shell it's a normal yukkuri. In post #14728 it says the shell weights it down so much it can't move properly, and underwater it'd be even harder.
The only yukkuri depicted as perfectly waterproof has been Nitori-types and Murasa-types (and they're both relatively new. Only the hats of aquatic Marisa have been demonstrated to be completely waterproof.
Shell Marisa have never been consistently portrayed for long enough to build consistent traits (save for the shell and speed concerns).
That's probably quite correct, but we were talking about how shell dissolved in the urine puddle that elder Marisa sprayed on her... from the stories that have mentioned it, yukkuri urine is basically sugar water or a sweet drink. It can't be the slightest bit acidic, as that would mean the yukkuri would dissolve itself.
Well, it's not quite correct to say that it can't be the slighest bit acidic; organic chemistry gets interesting, and I'm sure there are some sugars which are acidic when dissolved. Of course, the bigger problem here is the assumption that acidic == dissolves things, which is both an over-simplification, and false in many cases (otherwise we wouldn't be able to hold acid in anything).
I believe that's where we bring in PH scales. Anything under a 7 is technically acidic, even though you need to be around 5 or something to start causing damage to human tissue. Our blood PH isn't allowed to fluctuate a whole lot at all without messing something up (forget the exact number for blood)
Wikipedia says human blood is 7.35-7.45, so slightly basic... And actually, in terms of damage, I'd think alkali solutions would be a bigger immedeate thread. When it comes to flesh (and other organics), acids burn, bases melt. (This is one way that lye soap works; it is literally melting the top layer of skin off, taking the dirt with it; this is in addition to the lye's contribution to the creation of hydrophilic/phobic molecules, which do most of the cleaning. And of course, a sufficiently high or low pH will melt damn near anything.)
I think the reason that the shell Marisa died is because when it jumped off the table it may have been damaged, so when it got peed on the pee got into its paste.
If she did have any injury, it was probably the weight of the shell made the fall harder and bruised her foot.
But she didn't need to get piss in her paste or anything because she wet herself and got pissed on and then probably cried herself to sleep so she just naturally melted in the resulting puddle of water, which was what was implied.
Either that or the puddle melted her foot so that she couldn't move which caused her to melt anyway.
Ah, I just noticed, in the original Japanese, Shell doesn't say "Put cute Marisa on the table." She just says "Put Marisa on the table." The added "cute" makes her come off as a shithead when she actually isn't.